Catalysis is an initiation or a change in the rate of a chemical reaction due to the participation of a material called a catalyst. Catalysts that speed the reaction are known as positive catalysts. Catalysts that slow the reaction are known as negative catalysts, or inhibitors. Unlike reactants, a catalyst is not consumed by the reaction itself.
A catalyst works by providing an alternative reaction mechanism pathway from the reactants to the reaction product. The rate of the reaction is increased when this alternative route has a lower activation energy than the reaction route not mediated by the catalyst. Catalysts can also enable reactions that would otherwise be blocked or slowed by a thermodynamic or kinetic barrier. The catalyst may increase a reaction rate or selectivity of the reactants, or enable the reaction to proceed at lower temperatures than would otherwise be possible. As such, catalysts can be very valuable tools in industrial chemical processes.
There can nevertheless be drawbacks to the use of catalysts. For example, tin compounds are used extensively in industrial products such as coatings as catalysts for isocyanate/hydroxyl reactions. Amine compounds are used as catalysts for polyene/thiol reactions. Unfortunately, the levels of these catalysts required to provide acceptably fast cure rates and final product properties often result in a short application time window after the reactants are mixed.
There is thus a need to work in a timely manner so that the mixed components maintain a low enough viscosity for application to a substrate, e.g. by spraying. The span of time during which the coating is ready to apply to a substrate and still of low enough viscosity to be applied is commonly referred to as “pot life.”
Typically, pot life must be balanced with cure speed of the applied coating. For instance, in a multi-component coating system that uses a catalyst, the pot life and cure speed are primarily controlled by the amount of catalyst present. Accordingly, if a fast cure speed is required more catalyst can be used but that will also cause a shorter pot life. Conversely, if a longer pot-life is needed less catalyst can be used but the cure speed would also be retarded.
It is also important that the applied coating composition dry and harden quickly so that dirt pick-up is minimized and valuable shop space is not occupied with the coated substrate, such as an automobile, while it is drying. The length of time between when a coating is applied to a substrate and when the coating has dried or cured sufficiently that dust or other debris falling onto the coated substrate will not stick to the coated substrate is referred to as “dust-free time” or “tack-free time” and is an indicator of the speed of cure. One way to speed the drying and cure of the composition is to add additional catalyst, but this shortens the time available for processing, e.g. by spraying, since higher catalyst levels also cause the viscosity of the composition to increase more quickly as reaction rates increase.
Polyenes containing unsaturated groups, such as acryloyl groups, react with active hydrogen-containing compounds. Such a reaction is believed to involve an addition of an anion derived from the nucleophilic, active hydrogen-containing compound, acting as a donor, to an activated unsaturated group, which serves as an acceptor. When these active hydrogen-containing compounds are C—H compounds such as malonic ester or acetoacetate, the reaction is known as a Michael addition reaction. It is also known that SH-containing compounds may function as active hydrogen-containing compounds in a reaction mechanism which is similar to the Michael addition reaction. Such a reaction mechanism with SH-containing compounds is known as a thiolene reaction.
Catalysts for the thiolene reaction include quaternary ammonium compounds, tetramethyl guanidine, diaza-bicyclo-undecene, and diaza-bicyclo-nonene. Thiolene reactions catalyzed by these strong bases can be difficult to control and such reaction mixtures typically have a short pot life.
It would be desirable to catalyze chemical reactions between polyenes and thiols using catalysts that overcome these drawbacks of the prior art by lengthening the pot life of the composition and/or by accelerating the thiolene reaction rate after application without adversely affecting the pot life.